Fluid pressure transmission media



Patented May 14, 1940 UNITED STATES PATENT OFFICE FLUID PRESSURE TRANSMISSION MEDIA No Drawing. Application July 30, 1937, Serial- No. 156,521

6 Claims.

This invention provides improved compositions for use in fluid pressure transmission systems including hydraulic brake systems, lifts, checkvalves and the like. In particular it is concerned with a new pressure transmission composition containing as an essential ingredient a polyalkylene glycol dialkyl ether combined with a suitable non-mineral lubricating oil of low solidifying point. The dialkoxy compounds employed are those which have a maximum of two carbon atoms in either of the alkyl substituents. A particularly useful example which is illustrative of such compounds is tetraethylene glycol dimethyl ether.

A satisfactory hydraulic brake fluid must possess certain properties. For example, it must not corrode the metal surfaces which it contacts in the system; it must not becomeunduly viscous at 40 F., so that emcient operation of the moving parts is hindered or even prevented; it must not vaporize at temperatures of around 200 F., thereby causing separation of the components of the mixture with possible vapor locking of the system; and it must exhibit complete miscibility with castor oil and with other castor oil-containing fluids. This latter feature is of special importance in order that brake systems may be properly reserviced. An efiicient brake fluid must not cause swelling of the rubber members of the system to the extent of more than 6% or 7%. This is very important because the free movement of the rubber parts is vital to operation of the system. 1

Whenever these polyalkylene glycol diethers 35 are combined with a non-mineral oil of low solidifying point, such as castor oil, soy bean oil and the like, in proportions ranging from about 75% to about 10% by weight of the diether to from about 90% to about 25% by weight of the oil, 40 very satisfactory hydraulic brake fluids are obtained. Within the range noted, mixtures of about 50% by weight of tetraethylene glycol dimethyl ether and about 50% by weight of castor oil have been found to give exceptionally good results. These compositions may also contain small amounts of weakly alkaline materials which function as corrosion inhibitors and serve to reduce any tendency toward gum formation.

In these combinations, the maximum rubber swelling observed was 3.5%, and no noticeable corrosion of metal surfaces in the system was apparent. Another unusual feature of these fluids is their complete miscibility with castor oil and with other castor oil-containing fluids. The new liquids are sufllciently fluid at 40 F. to insure the operation of the braking system, and they do not unduly vaporize at temperatures as high as 200 F. While tetraethylene dimethyl ether is especially desirable in the practice of this invention, other compounds of this general class are 5 equally suitable. For example, tetraethylene glycol diethyl ether, or similar derivatives of pentaethylene glycol; tetraethylene glycol methyl ethyl ether; and, in general, the related polyalkylene glycol dialkyl ethers having no more than 10 two carbon atoms in the alkyl groups can be used. When tetraethylene glycol diethyl ether was compounded with castor oil in equal parts by weight a fluid was obtained which possessed similar desirable properties to that previously described. All of these compounds are characterized by a high boiling point, and by extreme chemical stability even at elevated temperatures.

While the above compositions provide exceptionally good pressure transmission media, fur- 20 ther additions can be made if desired. For example, diluents, or viscosity-reducing agents, may be used, such as the monoethers of monoand polyethylene glycols and certain monohydric alcohols. Mixtures of polypropylene glycols also may be included in the compositions. In this connection, those mixed products resulting from the reaction between equimolecular weights of propylene oxide and propylene glycol are especially referred to. Such products will contain 30 about 85% dipropylene glycol, the remainder consisting of tri-, tetraand pentapropylene glycols. These mixed polypropylene glycols can be used in amounts up to 35% by weight of the total composition, and because of their lubricating 35 quality can be substituted, in whole or in part, for the castor oil. If castor oil is to be employed, the percentage of added polypropylene glycols used may be minimized. A fluid composed of 50% of tetraethylene glycol dimethyl ether, 17% of castor oil and 34% of mixed propylene glycols, all by volume, is particularly effective. The rubber swelling induced by the fluid amounts to only 2.5%; it does not volatilize at temperatures of 200 F.; it is not so hydroscopic as to cause separation of the components during operation; and it is completely miscible with castor oil and with castor oil-containing compositions. Another fluid composed of 30% tetraethylene glycol dimethyl ether, 20% castor oil, 30% mixed propylene glycols, and 20% ethylene glycol monobutyl ether, likewise satisfied the rigorous demands imposed on this type of fluids. Other mixtures were prepared in which tetraethylene glycol dlethyl ether and tetraethylene glycol methyl ethyl ether were employed with similarly satisfactory results in the same amounts as the tetraethylene glycol dimethyl ethers. Other combinations may be made with varying proportions of the respective components within the ranges set forth above.

The compositions of this invention may contain small quantities of weakly alkaline materials, such as the ethanolamines or sodium and potassium arsenates, which will act as corrosion inhibitors. These alkaline materials additionally serve to reduce tendencies toward gum formation which might otherwise occur in the use of such unsaturated vegetable oils as soy bean oil and castor oil. Amounts of the alkaline materials from 0.1% to 1.0% by weight of the composition are usually suitable.

Modifications of the invention will be apparent, both as to the amounts and nature of the components used, and such modifications are included within the invention as defined by the appended claims.

I claim:

1. A hydraulic brake fluid containing as its essential ingredients from about 75% to about 10% by weight of dimethyl tetraethylene glycol combined with from about 90% to about 25% of castor oil.

2. A hydraulic brake fluid containing as its essential ingredients from about 75% to about 10% by weight of dimethyl tetraethylene glycol combined with from about 90% to about 25% of castor oil, together with a small amount of weakly alkaline material. l

3. A hydraulic brake fluid essentially composed of from about 75% to about 10% by weight of tetraethylene glycol dimethyl ether combined with from about 85% to about 10% of castor oil and from about 5% to about of a mixture of polypropylene glycols.

4. A hydraulic brake fluid essentially composed of from about to about 10% by weight of tetraethylene glycol dimethyl ether combined with from about to about 10% of castor oil, from about 5% to about 35% of a mixture of polypropylene glycols and from about 5% to about 20% of ethylene glycol monobutyl ether.

5. Fluid pressure transmission media comprising a substantial quantity of the dimethyl ether of tetraethylene glycol combined with a nonmineral lubricating oil of low solidifying point.

6. A hydraulic brake fluid essentially composed of from about 75% to about 10% by weight oi. the dimethyl ether of tetraethylene glycol combined with from about to about 25% of castor oil, and also combined with a viscosity reducing agent.

HARVEY R. FIFE. 

